What was wrong with Grace? *possible spoilers*

Grace was two characters meshed into 1. In the book Lily had Grace removed from the guest list at one of their social gatherings. That's why Grace told Aunt Julia about Lily's escapades. There was another character in the book named Gerty that was close to Lily but was in love with Selden, IIRc. There are some posts here I think.

"What happens to a dream deferred?"

If Grace seemed immature, so was Lily. Both terribly naive and ignorant of the world. This is the way women in the upper classes were raised in the Victorian era. They were hot house flowers, too delicate to be assaulted by the rude world. This left them horridly disadvantaged if circumstances changed for the worse.

Grace is horrible. But she's also a product of being raised by straightlaced Aunt Julia...so, I guess it makes sense. She's also a lonely spinster, so that's probably eaten away at her soul, too. She's hard and bitter.